AS THE BIOLOGIST SEES IT 



You can see the utility to the breeder of 

 silkworms if I am able to say to him that 

 if he will cross a silkworm moth of a 

 certain race which spins yellow silk with 

 one from a certain white-silk spinning 

 race and it makes no difference whether 

 the male or the female be either of the 

 white or the yellow silk race; there is no 

 factor of sex-potency in the outcome he 

 will get a progeny of silkworms all of 

 which spin yellow cocoons, but that if for 

 a second generation he mates two of these 

 yellow-spinners together he will get a 

 brood of which three-fourths will spin 

 yellow cocoons and one-fourth white 

 cocoons, while if for a third generation he 

 mates two of these white spinners together 

 he will get a brood all of which will spin 

 white, and only white cocoons, while if he 

 mates all of the yellow spinners inside 

 their group he will get from one-third of 

 these matings broods which spin nothing 

 but yellow cocoons but from two-thirds 

 of them broods which spin both yellow 

 and white cocoons in the precise propor- 

 73 



